Joints often undergo degenerative changes due to a variety of reasons. When joint degeneration becomes advanced or irreversible, it may become necessary to replace the natural joint with a prosthetic joint. Artificial implants, including hip joints, shoulder joints, and knee joints are widely used in orthopedic surgery. Specifically, hip joint prostheses are common. The human hip joint acts mechanically as a ball and socket joint, in which the ball-shaped head of the femur is positioned within the socket-shaped acetabulum of the pelvis. Various degenerative diseases and injuries may require replacement of all or a portion of a hip using synthetic materials, typically metals, ceramics, or plastics.
A standard hip replacement includes two bearing surfaces that form an interface between the femoral head and the acetabulum. The first bearing surface is typically part of a prosthesis shell or acetabular cup, which may be formed of metal, ceramic material. A liner (conventionally formed of a polymer such as ultra high molecular weight polyethylene, a ceramic material, or in some cases, a metal liner) is then fit tightly within the shell to provide an inner bearing surface that receives and cooperates with an artificial femoral head in an articulating relationship to accommodate the relative movement between the femur and the acetabulum.
The cup, or a cup and liner assembly, is typically fixed either by placing screws through apertures in the cup or by securing the cup with cement. In some cases, only a liner is cemented in a patient due to poor bone stock. In other cases, a cup having a porous surface may be press fit into an acetabular surface after the surface is prepared with a reaming tool.
Restoration of a patient's natural anatomy (e.g., restoring the normal center of hip rotation) is accomplished by acetabular preparation. Preoperative templating (e.g., preoperative X-rays including an anteroposterior (AP) view of the pelvis and hips, an AP view of the affected hip and femur, and a lateral view of the affected hip) is often used to properly reconstruct the hip joint.
Using the existing anatomy as the reamer guide, the patient's acetabulum is expanded concentrically using acetabular reamers of increasing diameter (e.g., in 1 mm to 2 mm increments) until a desired acetabular diameter is achieved, for example, when a subchondral bone is reached. Acetabular reaming is directed in approximately 45 degrees of abduction and 20 degrees of anteversion relative to the acetabulum for final position of the acetabular component. Once the patient's acetabulum has been reamed, the appropriate acetabular shell is selected, attached to the acetabular cup positioner/impactor, and inserted into the acetabulum.
With current systems, the acetabular shell is typically aligned to the patient's acetabulum using an external X-bar located above the operating table on which the patient is positioned. The X-bar is positioned so that the vertical bar of the X-bar is perpendicular to the long axis of the patient's body and the appropriate crossbar (e.g., left or right) aligns with the long axis of the patient's body. Vertical orientation of the X-bar and alignment of the appropriate cross bar with the body axis provides 45 degrees of abduction and 20 degrees of anteversion. The inserter is then firmly tapped with a mallet until the acetabular cup is fully seated.
With current systems, the use of a generalized X-bar positioning system is often insufficient to properly align the acetabular implant as a result of anatomical differences between patients or differences in patient position during surgery. Misalignment of the acetabular implant can result in leg length discrepancies, inadvertent lateralization of the hip center of rotation, or both. The surgeon will often have to perform a second or subsequent surgery (e.g., an acetabular revision surgery) to correct the misalignment. This can be time consuming and expensive, and may subject the patient to additional health risks.